To provide a transmission shaft superior in torsion fatigue characteristics, which has machinability equal to or more excellent than that made from a conventional Pb-added free-cutting steel without necessarily containing a component for free cutting such as Pb and without deteriorating cold and hot forgeability, and besides, uses a machine-structure carbon steel superior in properties after having been nitrided at least in a part.
The transmission shaft has a constant-velocity universal joint in one end of a stub shaft. The stub shaft is made from a steel material which contains a graphite phase in an amount of 5 mass% of the C content or more when the stub shaft is formed, and makes remaining graphite particles and holes originating from graphite, which exist in a surface layer part after the stub shaft has been induction-hardened and tempered, controlled into circle-equivalent diameters of 7 μm or smaller.
COPYRIGHT: (C)2007,JPO&INPIT
Kunikazu Tomita
Kimura
Yasuhiro Omori
Takaaki Toyooka
Kazuhiko Yoshida
ntn corporation
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